The role of surface antigens of lymphoid cells in determining inherited differences in the susceptibility of chickens to oncogenic viruses will be investigated as follows: Alloantigens (individually inherited antigens) on cells from the bursa of Fabricius and thymus, and on peripheral leukocytes derived from these organs (respectively B- or T -cells) will be identified and their mode of inheritance established. Immunoglobulin (Ig) alloantigens (allotypes) will be similarly studied, since it is known that Ig can be readily detected on B-cells. The association will be tested of specific B-cell alloantigens (including Ig alloantigens) with inherited differences in (a) susceptibility to infection by lymphoid leukosis and leukosis-sarcoma viruses; (b) the subsequent course of oncogenesis; and (c) associated pathological changes. Similar association tests will be made for T-cell alloantigens and Marek's disease herpes virus. The mode of action of lymphoid cell alloantigens will be examined as follows: pathological changes following virus inoculation will be studied in chicks of one viral susceptibility genotype which have had their bursal cell development inhibited, and have been repopulated with bursal cells from chickens of a contrasting viral susceptibility genotype. The specific interaction of alloantigens with viruses will be examined by adsorption-inhibition and radiochemical techniques. The possibility will be tested of utilizing antialloantigen sera to suppress the development in heterozygous chicks, of those bursal cells and derived B-cells which are susceptible to lymphoid leukosis virus.